Added sahlnaskh arabic font and give it higher precedence than DroidNaskh

sahlnaskh font is a fork of DroidNaskh font with some issues fixed.
Details of fixes are in https://github.com/khaledhosny/sahl-naskh

This should fix Jira issue: http://jira.omnirom.org/browse/OMNI-307

Change-Id: I9251eaf6c22288428f49916aa798be4ff3eaaead